1999 Volume 85 Issue 3 Pages 269-275
Constant load creep tests have been conducted at 873K under an initial stress from 50 to 70MPa for 9Cr-2Co steels containing 0.06, 0.11, 0.16 or 0.19 mass% nitrogen. An increase of nitrogen concentration decreases creep rate and increases rupture life. Except for an early and a final stage of creep, the logarithm of creep rate ε was expressed as a linear function of strain in the form;
lnε=lnε0+lnΩε,
where ε0is imaginary initial strain rate and Ω is strain acceleration factor defined in the text, respectively. The stress dependence of ε0 was expressed by nth power of applied stress. The stress exponent, n decreased gradually with an increase in nitrogen concentration from 5.0 to 3.9. It was found that the linear-type primary creep appeared in some creep conditions. An analysis based on the concept of threshold stress and the formation of solute atmosphere consisting of nitrogen-chromium atom pair revealed semi-quantitatively that dislocations move in a viscous manner dragging solute atmospheres behind them in creep deformation of the present high nitrogen ferritic steels.
